1. Field of the Invention
This invention relates generally to the method and device for charging a rechargeable battery. More particularly, this invention relates to an improved method and device for accelerating the rate of charging a rechargeable battery without causing rapid increase of battery temperature.
2. Description of the Prior Art
Conventional methods of increasing the rate for charging a rechargeable-battery to achieve rapid-charge still encounter several limitations. One of the major difficulties is the problem of battery overheating resulting from rapid temperature rise during the process of a rapid charge operation. A large current flowing through the battery causes temperature to rise. This is due to the heat produced from a conversion of the electrical energy to a thermal energy in the form of "IR" where "R" represents the internal resistance of the battery, and "I" represents the charging current. With an increase in the charging current, larger amount of heat is produced. A potential damage to battery that may result from overheating caused by rapid temperature rise becomes a major concern. Particularly, when larger current is employed to rapidly charge the battery for the purpose of shortening the charging cycles.
A typical battery charging system is disclosed by Theobald et al. in U.S. Pat. No. 5,510,693 entitled "Method of Battery Charging" (Issued on Apr. 23, 1996) a battery charging process. Three charging rates are applied. The initial rate of a one-capacity charging rate, i.e., a charging rate C, for charging the battery until the temperature of the battery reaches a certain level. The charger then changes to a trickle charge rate of C/8 to finish the charging of the battery. Then the charger reduces the charging rate to C/40 to maintain the battery's charge. The process then monitors the voltage of the battery. If the voltage indicates that the battery is being discharged, then the charging rate is returned to a trickle-charging rate of C/8. The length of time required to charge the battery to the full capacity may take one hour or even longer when a charging rate C is applied. Such a system and process would not able to satisfy a rapid charging requirement.
A common technique to rapidly charge the batteries by applying a charging system as disclosed by Theobald et al. is to charge it with a large constant current A voltage detecting circuit is attached to the battery to monitor the rate of battery voltage increase, i.e., .DELTA.V/.DELTA.A.DELTA.T . Where .DELTA.V represents the voltage variation and A.DELTA.T represents a unit-length of charging time. The voltage detecting circuit then determine a point in time when .DELTA.V/A.DELTA.T is zero. At that point, the battery is fully charged. There is a tendency for the voltage to decrease slightly because usually the battery would be overcharged when a large current is applied. The charging operation is terminated based on a determination of the rate of change of .DELTA.V when it is detected that .DELTA.V/A.DELTA.T is zero.
This technique has several limitations. The first limitation is the length of time it requires to fully charge the batteries. Due to the concern of battery overheating, typically, the charging current is limited to a magnitude of "1C". Suppose that the full capacity of a battery is 1200 mah, i.e., 1200 millie-ampere-hour, then the "1C" charging current is 1.2 amperes. Approximately one hour would be required to fully charge the battery when a 1C-charge current is applied. In order to accurate detect the time when .DELTA.V/A.DELTA.T becomes zero, a high precision detection circuit is required. The voltage variations for a typical 1.2V battery over the charging time are approximately 50mV to 150mV. In order to prevent overcharge, it is necessary to apply more expensive and high precision detection circuits. Even with a high precision detection circuit, it is often inevitable that the battery may be overcharged. This overcharge phenomenon is caused by a delay of .DELTA.V/A.DELTA.T response. When charged by a large current, the .DELTA.V response of the battery tend to delay two to three minutes in reaction to the large charge current even that the battery is fully charged. The battery is under an overcharge condition in these two to three minutes. The life of the batteries is shortened when continuously charged by applying a .DELTA.V/A.DELTA.T technique due to this overcharge phenomenon. In order to avoid the overcharge, the charge current may be terminated slightly earlier before .DELTA.V/A.DELTA.T becomes zero. However, such practice sacrifices the capacity of the batteries to exchange for the benefits of rapid charge. More frequent charging operations would become necessary become the battery is operated under their full capacity. The life of the batteries again may be adversely affected due to more frequent charging operations.
In order to overcome these difficulties, many techniques are disclosed in prior art patents to improve the rapid charging performance. In U.S. Pat. No. 5,311,113, entitled "Method of Setting Peak-Timer of Electric Charger", Kojima discloses a method for charging a battery by applying a rapid charge voltage level and a trickle charge voltage level. The charger is provided with a total timer for measuring a total elapse-time starting from the initiation of a charging operation. The charger further includes a peak-timer for measuring an amount of elapse-time since the peak voltage is reached during rapid charging of the battery. An arithmetic circuit is employed in the peak-timer for determining the elapse-time since the peak voltage is reached as a function of the total elapse-time and the charging rate. This method of setting peak-timer of an electric charger may provide more accurate control of adjusting the time after the peak voltage is reached. However, a rapid charging operation may still limited by the difficulties that long charging time is required. Also, it is not clear whether a measurement of the peak-timer, as disclosed in this patent, can assist a charging operation to prevent battery damages caused by high temperature, especially during a rapid charging process. Using this patented peak-timer does not solve the difficulties faced by those of ordinary skill in the art for rapidly charging a battery.
Sage discloses in another U.S. Pat. No. 5,663,574, entitled "Pulse-Charge Battery Charger" (issued on May 27, 1997), a pulse-charge battery charger for charging nickel cadmium and nickel-metal hydride batteries. The charger includes a power supply system to provide full wave rectified unregulated DC volt power source and regulated 5 DC volt power source. The charger further includes a time control circuit and a temperature detection unit. A typical charging cycle is described as 1000 milliseconds of charging; 2 milliseconds of no charging; 5 milliseconds of discharging; 10 milliseconds for a second no charging period. The charging cycle is performed until the battery is fully charged. The charger, as disclosed, implements complicated temperature and time control scheme by constantly monitoring and feeding back the voltage, temperature, charging time and charging current variations to adjust the charging cycles. Due to the fact that the charging operations are relying heavily on the controlling and monitoring circuits implemented in the charger, expensive and high precision circuits are required to carry out such charge operations. Additionally, the overheating and overcharge problems associated with the .DELTA.V/A.DELTA.T techniques may still occur when the charge operation is dependent on the measurements of the condition of a battery when there is delay of the battery in reaction to the charging process.
Therefore, a need still exists in the art of rapid charging of a battery to provide a simplified and effective method to overcome the above discussed problems and difficulties. It is desirable that such methods should be simple and practical. It is further desirable that the method does not depends on high-precision and expensive measuring circuits such that rapid charging operations can be carried out economically in regular daily operations.